In many applications, including but not limited to electronic circuits used in satellites, it is important to achieve both (1) a high degree of compactness and (2) very long-range (measured in years) reliability despite extremely severe environmental conditions.
Circuit compactness, factor (1) stated above, is not only a function of the physical sizes of the components in the circuit, but is also a function of how far the components must be spaced from each other in order to prevent adverse interactions therebetween. Such size and spacing considerations are especially significant relative to high-voltage resistors--which often seem to be about the last things that circuit designers try to cram into a circuit of given physical size. The smaller the high-voltage resistors, the smaller the spaces into which they can be inserted without causing them to be intolerably close to other circuit components.
As indicated above, smallness of the resistors is not the only thing. Also of major importance is the tendency of high-voltage resistors to generate coronas that can adversely impact not only on adjacent components but on the resistors themselves. Despite the fact that the circuits are conventionally potted in materials, for example epoxy compounds, having high dielectric strengths, the corona discharges from the high-voltage resistors can have devastating and unpredictable consequences.
Factor (2) stated above, namely long-range reliability, is--vis-a-vis high-voltage resistors--largely related to the corona problems. The coronas can cause erratic tunneling through the dielectric from resistors to other circuit elements, or from one part of a resistor to another part thereof. The "tunnels" are minute in diameter, and can grow progressively over time until breakdown occurs.
The corona problem is especially crucial when the high-voltage resistors are incorporated in satellites. The extreme, repeated thermal shocks present in the satellites mean that high-strength dielectrics cannot practically be caused to engage the resistive film material itself. Any such engagement would, because of differences in coefficients of thermal expansion, tend to set up adverse shear forces in the resistive film. Thus, in satellite circuits, as elsewhere, layers of environmentally-protective buffer material (preferably silicone conformal) are placed around the resistive film, buffering it from the surrounding epoxy or other dielectric substance.